1. Field of the Invention
The present invention relates to ultrasonic imaging apparatus. More specifically, the present invention relates to an ultrasonic imaging apparatus which is used for imaging blood vessels of a subject.
2. Background Information
Angiography, or the mapping of blood vessels within subjects under examination, has been performed with a number of apparatus in prior art techniques. Typically, angiography in prior art applications have been performed with invasive techniques such as x-ray angiography, nuclear medicine, or surgery. For example, some prior art techniques of angiography are performed by injecting an x-ray opaque dye into a subject under examination allowing a period of time to transpire so that the dye becomes circulated within the blood stream of the subject and exposing the subject to ionizing radiation (e.g., x-rays) in order to image blood vessels in the subject. This is known as contrast injections. Other passive noninvasive techniques such as magnetic resonance imaging (MRI) may also be used. Of course, such techniques suffer from many disadvantages. For example, x-ray techniques require that the subject be exposed to ionizing radiation, which may be undesirable in certain circumstances, such as during pregnancy or where exposure to radiation is otherwise desired to be avoided. Techniques such as MRI require that the subject remain still for an extended duration and require the use of very expensive apparatus. Thus, them is needed a methodology to allow angiography without disadvantages of prior art, such as the use of invasive techniques such as x-ray angiography, nuclear medicine, or surgery.
Pulse echo ultrasonic imaging technology is one typically used for examining the internal structure of living organisms, such as blood flow. In the diagnosis of various medical conditions, it is often useful to examine soft tissues and/or blood flow within the body to show structural details of organs and blood vessels in these organs. Experienced clinicians can use this information in diagnosing various pathologies.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties of the echoes (e.g., amplitude and phase) from targets at varying depths within the body. Typically, the ultrasound beam is focused at various depths within the body in order to improve resolution or image quality. The echoes are received by a transducer, typically, the same transducer used for transmission and process to generate an image of an object, usually referred to as a B-scan image.
Measuring and imaging blood flow (or other fluid flow) in the human body is typically done using the Doppler principle, wherein a transmitted burst of ultrasound at a specific frequency is reflected from moving blood cells thereby changing the frequency of the reflected ultrasound in accordance with the velocity and direction of the flow.
The frequency shift of the reflected signals with respect to the transmitted signals may be detected, and since the amount of shift (or the Doppler shift) is proportional to the blood flow velocity, it may be used to display velocity information of blood flow on a video screen for imaging a living patient. However, typical prior art color Doppler imaging has been unsatisfactory for performing angiography because it displays velocity information, whereas, angiography requires information be displayed which is direction, angle, and velocity-independent. Moreover, angiography requires high contrast between tissue and blood vessels in a subject to determine the location of blood vessels. In prior art Doppler color imaging, the flow displayed upon the display was shown in relation to its movement towards or away from the probe, typically represented in distinguishing colors, such as red and blue (reflecting the red and blue shift of the Doppler signal data due to movement to or away from the transducer). However, such information is unneeded in angiography, which seeks to merely display blood vessels within a living organism. In addition, the prior art aliases the image making blood vessels look inhomogenous or discontinuous. The prior art is also limited in that noise in an ultrasonic imaging system appears to be flow.
Thus, it is desired to use ultrasonic imaging for performing angiography in living subjects, however, prior art systems have been inadequate for this application.